1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device fabricated by using three masks and a fabrication method thereof, and more particularly, to an LCD device fabricated using three masks by a process that uses amorphous silicon as a channel layer.
2. Discussion of the Related Art
An LCD device includes an active element such as a thin film transistor (TFT) in each pixel region to drive each pixel. The driving method of the LCD device is commonly called an active matrix driving method. In the active matrix type driving method, the active elements are disposed in pixels arranged in a matrix form to drive the pixels.
FIGS. 1 and 2 show a related art active matrix type TFT LCD device. As shown in FIG. 1, the related art TFT LCD device uses TFT 110 as the active element. In the TFT LCD device, the N×M number of pixels are arranged on a first substrate 201 (FIG. 2) and the TFT 110 is formed at respective intersections of gate lines 101 and data lines 102. The gate lines 101 delivers scanning signals applied from an external driver and the data lines 102 delivers image signals.
The TFT 110 includes a gate electrode 106 connected with the gate line 101, a semiconductor layer 103 formed on the gate electrode 106, and source and drain electrodes 104 and 105 formed on the semiconductor layer 103. The scanning signals applied through the gate lines 101 activate the semiconductor layer 103. A pixel electrode 120 is formed at a display region of the pixel, which is connected with the source electrode 104 and the drain electrode 105 to receive an image signals when the semiconductor layer 103 is activated.
The structure of the related art TFT LCD device will be described in detail with reference to FIG. 2. FIG. 2 is a sectional view of the related art TFT LCD device taken along line I-I of FIG. 1. With reference to FIG. 2, the TFT 110 is formed on a first substrate 201 made of a transparent material such as glass. The TFT 110 includes a gate electrode 106 formed on the first substrate 201, a gate insulation layer 203 formed on an entire surface of the first substrate 201 including the gate electrode 106 formed thereon, a semiconductor layer 103 formed on the insulation layer 203, source and drain electrodes 104 and 105 formed on the semiconductor layer 103, and a passivation layer 204 formed on the entire surface of the first substrate 201 including TFT 110. Accordingly, an array substrate 210 is defined having above described components.
A pixel electrode 120 is formed on the passivation layer 204 and is connected with the drain electrode 105 of the TFT 110 through a contact hole 107. The contact hole 107 is defined through the passivation layer 204.
A color filter substrate 220 facing the array substrate 210 includes a second substrate 202 made of a transparent material such as glass, a black matrix 205 formed on the second substrate over the TFT and over non-display regions defined between the pixel regions, and a color filter layer 206 to implement actual color. The black matrix 205 prevents transmission of light to the non-display regions. A common electrode 207 for providing an electric field to the liquid crystal layer 240 can be additionally formed on the color filter layer 206. When the color filter substrate 220 and the array substrate 210 are attached, a liquid crystal layer 240 is formed therebetween to complete the related art TFT LCD device.
The related art TFT LCD device is fabricated through a complicated process such as a photolithography process using masks. The related art method for fabricating the TFT LCD device will be described with reference to FIGS. 3A through 3E.
As shown in FIG. 3A, a metal layer is stacked on the entire surface of the first substrate 201, on which photoresist is coated, and then a gate line (not shown) and a gate electrode 106 connected with the gate line are formed by performing the photolithography process.
Next, with reference to FIG. 3B, a gate insulation layer 203, a semiconductor layer 103a and an ohmic contact layer 211 are sequentially formed on the entire surface of the first substrate 201 with the gate electrode 106 formed thereon. Subsequently, a photosensitive film 230 is coated on the ohmic contact layer 211, and then the photolithography process is performed to form an active pattern. The active pattern includes a stack structure of the semiconductor layer and the ohmic contact layer.
Next, as shown in FIG. 3C, a conductive layer 212 is formed on the entire surface of the first substrate 201 and a photosensitive film pattern 231 is formed on the conductive layer 212. Then, source and drain electrodes 104 and 105 are formed using the photosensitive film pattern 231 as a mask.
As shown in FIG. 3D, when the conductive layer 212 is patterned to form the source and drain electrodes 104 and 105, a channel region is defined through the conductive layer 212 and the ohmic contact layer 211 through etching process. In addition, the channel region is defined at a middle portion of the active pattern.
As shown in FIG. 3E, a passivation layer 204 can be additionally formed on the first substrate 201 to protect the TFT. Photoresist is formed on the passivation layer 204 and the photolithography process is performed to form a contact hole 107 exposing a portion of the drain electrode 105. A transparent material such as indium tin oxide (ITO) is formed on the passivation layer 204 and within the contact hole 107 and then etched to form the pixel electrode 120.
Although not shown, after the black matrix 205 and the color filter layer 206 are formed on the second substrate 202, the first and second substrates are attached and a liquid crystal layer is filled therebetween to complete the related art TFT LCD device.
As mentioned above, in order to fabricate the related art TFT LCD device that uses the semiconductor layer as the channel layer, the plurality of masking processes are required. For example, in the related fabrication process of the TFT LCD device, five masking processes are performed to form the TFT, and six masking processes are performed to form up to the pixel electrode.
However, the related art fabricating method is an expensive process, since every time a mask process is introduced, several supplementary processes are required, thereby increasing a total fabrication cost. In addition, etching process discharges environmentally harmful materials. Therefore, efforts for reducing a total number of masks in fabricating the TFT LCD devices are actively pursued.